Why do I keep seeing "this iphone is open in another window"?

Hi curly1,

Thanks for posting in Apple Support Communities.

It sounds like you're getting an iPhone error on your Mac while it's connected. To address the behavior, force close Finder and relaunch it.

Force an app to quit

Press these three keys together: Option (or Alt), Command, Esc (Escape).1 Or choose Force Quit from the Apple menu  in the corner of your screen.

A Force Quit window opens. Select the app that you want to quit, then click Force Quit:

The Finder is always open, but if it stops responding, you can force it to quit and then open again: Select Finder in the Force Quit window, then click Relaunch. 

How to force an app to quit on your Mac - Apple Support

Regards.

Hello curly1,

If it's still not working, try starting the computer in safe mode.

Start up your Mac in safe mode

If the phone is still not accessible, make sure the computer is up to date: Update macOS on Mac then contact Apple to report the issue: Get Support

Cheers!